Spread spectrum is a technique permitting multiple signals to share a common frequency space. Individual signals sharing this space are separated from one another by a spreading code so that each signal while sharing a common frequency preserves its own information (i.e., and channel identity) by means of the spreading code applied to it. It is found to be desirable in some systems to use sets of spreading codes for each channel that are orthogonal to one another to reduce inter-channel interference during simultaneous transmissions.
Another desirable characteristic of the spreading codes is their ability to produce a “white” spectrum, which is to say the signal power is evenly distributed over the spectrum occupied by the signal, or that the signals approximate the spectrum of “white” noise. Whitening techniques are known to exist which whiten a given set of spreading codes so that for each user they approximate white noise with the desired bandwidth. These known techniques include overlaying a pseudo-noise (PN) sequence onto the SS code. Another technique is a code hopping technique where the individual users are periodically assigned differing codes (i.e., code swapping) from a code set used by all the users.